Tube assembly

ABSTRACT

A tube assembly that may be for a fuel nozzle of a fuel system of a gas turbine engine may have a first tube defining a first flowpath along a centerline, a second tube generally spaced radially outward from the first tube with a first void located between and defined by the first and second tubes, and a support structure located in the first void and extending between the first and second tubes. The support structure is constructed and arranged to minimize or eliminate thermal conduction between the tubes. The entire assembly may be additive manufactured as one unitary piece. One example of a method of operation may include designed-for breakage of the structural support due to then al stresses thereby further minimizing thermal conduction between tubes.

This application claims priority to U.S. patent application Ser. No.62/020,613 filed Jul. 3, 2014.

BACKGROUND

The present disclosure relates to a tube assembly, and more particularlyto an additive manufactured tube assembly.

Manufacturing of tube assemblies such as those containing tubes withintubes (or concentrically located tubes), as one example, require themanufacture of several individual parts then assembly to create thefinal product. In some examples, air within an annular void definedbetween the two concentrically located tubes acts as a thermal insulatorfor fluid that may be flowing through the inner tube. Sealing of thisvoid (i.e. complete encapsulation) to enhance the thermal properties ofthe surrounding air is difficult from a manufacturing perspective andnot typically accomplished, and if such were accomplished, it wouldrequire yet further parts thus limiting feasibility. Similarly,concentrically located tube may require support structures to maintainorientation between the tubes, such structures are intricate anddifficult to manufacture. Moreover structures connecting the tubes willcontribute toward unwanted thermal conduction between tubes.

Furthermore, there exist needs in various industries to reduce thenumber of manufactured parts for tube or conduit-like assemblies,thereby providing more robust and simpler designs requiring lessmaintenance, reducing manufacturing time and costs, improving thermalbarrier characteristics, and/or reducing thermal conduction pathsbetween inner and outer tubes of the assemblies, amongst others.

SUMMARY

A tube assembly according to one, non-limiting, embodiment includes afirst tube defining a first flowpath along a centerline; a second tubegenerally spaced radially outward from the first tube, and wherein afirst void is located between and defined by the first and second tubes;and a first sacrificial support structure located in the first void andextending between the first and second tubes that is constructed andarranged to break when under normal operating stress thereby limitingthermal conduction between the first and second tubes.

Additionally to the foregoing embodiment, the first tube, the secondtube and the support structure are additive manufactured as one unitarypiece.

In the alternative or additionally thereto, in the foregoing embodiment,the first void is an insulating void.

In the alternative or additionally thereto, in the foregoing embodiment,the assembly includes a third tube generally spaced radially outwardfrom the second tube, and wherein a second flowpath is located betweenand defined by the third and second tubes.

In the alternative or additionally thereto, in the foregoing embodiment,the assembly includes a fourth tube generally spaced radially outwardfrom the third tube, and wherein a second void is located between anddefined by the fourth and third tubes.

In the alternative or additionally thereto, in the foregoing embodiment,the assembly includes a second sacrificial support structure located inthe second void.

In the alternative or additionally thereto, in the foregoing embodiment,the second void is an insulating void.

In the alternative or additionally thereto, in the foregoing embodiment,the first void is under a negative atmospheric pressure.

In the alternative or additionally thereto, in the foregoing embodiment,the first and second cavities are under a negative atmospheric pressure.

In the alternative or additionally thereto, in the foregoing embodiment,the sacrificial support structure has a thickness within a range of0.004 to 0.008 inches.

In the alternative or additionally thereto, in the foregoing embodiment,the assembly includes a second sacrificial support structure located inthe second flowpath.

In the alternative or additionally thereto, in the foregoing embodiment,the tube assembly is at least part of a fuel nozzle for a gas turbineengine.

A tube assembly according to another, non-limiting, embodiment includesa first tube defining a first flowpath along a centerline; a second tubegenerally spaced radially outward from the first tube by a radialdistance, and wherein a first void is located between and defined by thefirst and second tubes; and a first support structure located in thefirst void, and extending between and engaged to the first and secondtubes, and wherein the support structure has a length greater than theradial distance for minimizing thermal conduction between the first andsecond tubes.

Additionally to the foregoing embodiment, the support structuresubstantially lies with an imaginary axial plane with respect to thecenterline and has at least one bend.

In the alternative or additionally thereto, in the foregoing embodiment,the first support structure is one of a plurality of first supportstructures spaced circumferentially from one-another.

In the alternative or additionally thereto, in the foregoing embodiment,the first support structure extends both axially and radially inwardfrom the second tube to the first tube.

In the alternative or additionally thereto, in the foregoing embodiment,the assembly includes a third tube generally spaced radially outwardfrom the second tube, and wherein a second flowpath is located betweenand defined by the third and second tubes; a fourth tube generallyspaced radially outward from the third tube, and wherein a second voidis located between and defined by the fourth and third tubes; and asecond support structure located in the second void.

In the alternative or additionally thereto, in the foregoing embodiment,the tube assembly is additive manufactured as one unitary piece.

A method of operating a fuel system of a gas turbine engine according toanother, non-limiting, embodiment includes the steps of flowing a firstfluid through a first flowpath defined by a first tube; exposing asecond tube surrounding and spaced radially outward from the first tubeto a first thermal gradient; and breaking a first sacrificial supportstructure located between the first and second tubes via thermal stressinduced by the first thermal gradient.

Additionally to the foregoing embodiment, the method includes the stepsof flowing a second fluid through an annular second flowpath definedbetween the second tube and a third tube that is spaced from the secondtube; exposing the third tube to a second thermal gradient; and breakinga second sacrificial support structure located between the second andthird tubes via thermal stress induced by the second thermal gradient.

The foregoing features and elements may be combined in variouscombinations without exclusivity, unless expressly indicated otherwise.These features and elements as well as the operation thereof will becomemore apparent in-light of the following description and the accompanyingdrawings. It should be understood; however, that the followingdescription and figures are intended to be exemplary in nature andnon-limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features will become apparent to those skilled in the art fromthe following detailed description of the disclosed non-limitingembodiments. The drawings that accompany the detailed description can bebriefly described as follows:

FIG. 1 is a cross section of a combustor of a gas turbine engineillustrating a fuel nozzle as an example of a tube assembly of thepresent disclosure;

FIG. 2 is a cross section of the tube assembly;

FIG. 3 is a cross section of a second embodiment of a tube assembly;

FIG. 4 is a cross section of a third embodiment of a tube assembly;

FIG. 5 is a flowchart of a method of operating the tube assembly; and

FIG. 6 is a schematic of an additive manufacturing system used tomanufacture the tube assembly.

DETAILED DESCRIPTION

FIG. 1 illustrates a fuel nozzle for a gas turbine engine as one,non-limiting, example of a tube assembly 20. The fuel nozzle 20 is partof a combustor 22 that may be annular in shape and concentricallydisposed to an engine axis A. The combustor 22 may further include abulkhead assembly 24, an outer wall 26, an inner wall 28, and a diffusercase module 34. The outer and inner walls 26, 28 project axially in adownstream direction from the bulkhead assembly 24, and radially definean annular combustion chamber 30 therebetween. An annular cooling plenum32 is generally defined radially between the outer diffuser case module34 and a diffuser inner case 36 of the engine. The bulkhead assembly 24and walls 26, 28 are located in the cooling plenum 32 immediatelydownstream from a compressor section 38, and upstream from a turbinesection 40 of the engine.

The annular bulkhead assembly 24 may extend radially between and issecured to the forward most ends of the walls 26, 28. Assembly 24generally includes an annular hood 42, a wall or heat shield 44 thatdefines the axial upstream end of the combustion chamber 30, and aplurality of swirlers 46 (one shown) spaced circumferentially aboutengine axis A and generally projecting or communicating through the wall44. A plurality of circumferentially distributed hood ports 48accommodate a respective plurality of the fuel injectors or nozzles 20as well as direct compressed air C into the forward end of thecombustion chamber 30 through the associated swirler 46.

The bulkhead assembly 24 introduces core combustion air into theupstream end of the combustion chamber 30 while dilution and cooling airis introduced into the combustion chamber 30 through the walls 26, 28and from the plenum 32. The plurality of fuel nozzles 20 and respectiveswirlers 46 facilitate the generation of a blended fuel-air mixture thatsupports combustion in the combustion chamber 30.

Each fuel nozzle 20 may receive fuel from at least one fuel manifold 50generally located radially outward of the case module 34. The elongatedfuel nozzle 20 may substantially extend longitudinally along acenterline 52 and in a radial inward direction with respect to theengine axis A, through the case module 34 and into the plenum 32. Thecenterline 52 and thus the nozzle 20 may then bend (i.e. see bend 54)and thus project in an axial downstream direction, extending through thehood port 48 and into the swirler 46 where fuel is then dispensed andatomized from the nozzle 20.

Referring to FIG. 2, the tube assembly 20 (i.e. a simplified fuel nozzlein the present example) may have a first or inner tube 56 co-extendingwith and surrounded by (e.g. concentrically located) a second or outertube 58. The inner tube 56 may define a flowpath 59 for the flow of afluid such as liquid fuel. The outer tube 58 may be spaced radiallyoutward from the inner tube 56 thereby defining a substantially annularvoid 60, there-between. Void 60 may generally be sealed (i.e. completelyencapsulated) from the plenum 32 and/or surrounding environment to actas a thermal insulator for any fluid (see arrow 62) flowing through theinner tube 56. To enhance the thermal insulating properties, the void 60may be under a negative atmospheric pressure and may further contain aninert gas such as nitrogen (N₂), Argon or any other gas compatible withthe material composition of the surrounding structures. Although liquidfuel in the present example, it is contemplated and understood that thefluid 62 may also be a gas, liquid such as oil and water, or even asolid material (e.g. powder) capable of flow. It is further understoodthat the term “tube” also refers to conduits, casings, pipes and otherstructures capable of fluid flow and/or encasement of a thermalinsulating gas.

Such fuel nozzles 20 flowing liquid fuel and operating in hotenvironments like the plenum 32, where temperatures may exceed 1,700degrees Fahrenheit (927 degrees Celsius), are susceptible to fuelvarnishing and coking due to high temperatures of more traditional fluidbearing tube(s). This coking can lead to decreased flow capacity of thenozzle and decreased quality of fuel delivery. To manage the temperatureof the tube 56 and thus the fluid or fuel 62 and prevent coking, thevoid 60 may be employed to substantially break the thermal conductionpath from the hot external environment to the inner tube 56. It isfurther contemplated and understood that other portions of a fueldelivery system of the gas turbine engine may utilize the same type ofassembly 20. For instance, the fuel manifold 50 may be susceptible tosimilar coking issues leading to unintentional mal-distribution of fuelin the system, and thus benefit from the same means of insulating a tubethat bears fluid flow.

The inner and outer tubes 56, 58 may each have at least one respectivebend 64, 66 that generally corresponds with the bend(s) 54 of thecenterline 52 and such that the void 60 is generally maintained (i.e.spacing between tubes). The bends 64, 66 may be orientated wherelongitudinal insertion of the inner tube 56 into the outer tube 58 (andif the tubes were separate pieces) is difficult if not impossible. Withsuch fitting difficulties, additive manufacturing the tubes 56, 58generally together and/or simultaneously and as one unitary piece, isadvantageous. Alternatively, each tube may have multiple bends along thecenterline 52 that may be directed in different directions, thismultiple bend configuration would also make fitting or insertion of theinner tube 56 into the outer tube 58 difficult, if not impossible,therefore, additive manufacturing the tubes simultaneously isadvantageous.

The fuel nozzle 20 may include a plurality of support structures 68 inthe insulating void 60 for properly locating the inner tube 56 withrespect to the outer tube 58 at least during manufacture. Each supportstructure extends between the inner and outer tubes 56, 58 forcontacting both tubes, and may be engaged (i.e. attached) to one of thetubes or both. The number and positioning of the support structures 68are dictated by the size and shape of the fuel nozzle 20 and may beaxially aligned and circumferentially spaced from one-another. Eachsupport structure 68 may be a pylon, a foil, or a relatively thin sheetand may be manufactured of the same material of at least one of thetubes 56, 58. Alternatively or in addition thereto, the supportstructure 68 may have a greater porosity than either tube 56, 58 or maybe made of a different material (e.g. ceramic) to reduce thermalconductivity between tubes.

Whether the support structure 68 is a pylon, a thin sheet or some otherform, the support structures 68 may include a support structure 68A witha bended or serpentine profile that has an overall length that isgreater than a radial distance 70 between the inner and outer tubes 56,58 (also see FIG. 3). That is, at least one of the support structures68A may lie in an imaginary plane that is normal to the centerline 62yet has a length that is greater than the radial distance 70 orthickness of the void 60. The support structures 68 may further includesupport structures 68B where each support structure 68B contacts bothtubes 56, 58, but also extends in both an axial and radial direction,thereby having a length that is greater than the radial distance 70.Because the longitudinal lengths of the support structures 68A, 68B islonger than the proximate radial distance 70, thermal conduction throughthe structures 68A, 68B (e.g. heat transferred from the outer tube 58 tothe inner tube 56) is reduced or minimized. Moreover, the engagement orcontact area of each support structure 68 to one or both tubes 56, 58(e.g. the outer tube 68 in the present nozzle example) is minimized toreduce thermal conduction while maintaining the structural needs of theassembly or nozzle 20.

Manufacturing of such a complex and intricate tube assembly 20 (with thesupport structures 68) may be simplified by applying an additivemanufacturing process. The thickness or diameter of the supportstructure 68 may be as small as about 0.004 inches (0.102 millimeters)and/or the minimal production capability of the additive manufacturingprocess applied. The tube assembly 20, or portions thereof, may beadditive manufactured as one unitary and homogenous piece. Materialcompositions include, but are not limited to, nickel (e.g. INCONEL 718,625), Waspaloy® (of United Technologies Corporation), Stellite® (of theDeloro Stellite Company), titanium, steels and stainless steels, cobalt,chrome, Hastalloy® X (of Haynes International Corporation), and others.

Referring to FIG. 3, a second embodiment of a tube assembly isillustrated wherein like elements have like identifying numerals exceptwith the addition of a prime symbol. The tube assembly 20′ of the secondembodiment has a flowpath path 59′ defined by an inner tube 56′. Asecond tube 58′ is spaced radially outward from the inner tube 56′ witha substantially annular first void 60′ defined there-between. A thirdtube 72 may be spaced radially outward from the second tube 58′ with anannular second flowpath 74 defined there-between. A fourth tube 76 maybe spaced radially outward from the third tube 72 with a substantiallyannular second void 78 defined there-between.

One or both of the voids 60′, 78 may have thermal insulating propertiesand may contain a plurality of support structures 68′ (e.g. structures68A′ illustrated). The flowpaths 59′, 74 may flow the same or differentfluids and in the same or opposite directions. For example, flowpath 59′may flow liquid fuel, and flowpath 74 may flow air or gas. It is furthercontemplated and understood that additional tubes may be added and theradial orientation of the voids with the flowpaths may be altered.Moreover, the outer or fourth tube 76 may not be generally tubular, butmay take the form of an aerodynamic shape to complement flow around theexterior of the tube assembly; for example, flow in the plenum 32 (seeFIG. 1).

Referring to FIG. 4, a third embodiment of a tube assembly isillustrated wherein like elements have like identifying numerals exceptwith the addition of a double prime symbol. The tube assembly 20″ of thethird embodiment has a flowpath path 59″ defined by an inner tube 56″. Asecond tube 58″ is spaced radially outward from the inner tube 56″ witha substantially annular first void 60″ defined there-between. A thirdtube 72″ may be spaced radially outward from the second tube 58″ with anannular second flowpath 74″ defined therebetween. The annular void 60″may flow a fluid or may serve as a thermal insulating layer. Similarly,the flowpath 74″ may alternatively be an insulating void.

The tube assembly 20″ may further include at least one sacrificialsupport structure 68″ designed to break when placed under stress (e.g.stress created during normal operation of the assembly 20″), and therebyminimize or eliminate any thermal conduction between tubes. Thesacrificial support structures 68″ may be in one or both of the void 60″and the flowpath 74″. Each structure 68″ may have and extends between abase portion 80 and a breakable portion 82. The breakable portion 82 maybe engaged to the tube that generally experiences the greatest thermalgradient. For instance and in the present example, the assembly 20″ maybe in a hot surrounding environment and it may be desired to keep thefluid flowing in flowpath 59″ cool. In this thermal orientation, thebreakable portion 82 of the structure 68″ in the flowpath 74″ may beengaged to the tube 72″, and the breakable portion 82 of the structure68″ in the void 60″ may be engaged to the tube 58″. The thickness ordiameter of the breakable portion 82 may be within the range of about0.004 inches to about 0.008 inches (102 to 203 microns). As describedbefore, such complex and intricate assemblies and/or structures may beadditive manufactured.

Referring to FIG. 5, a method of operating a fuel nozzle 20″ of a fuelsystem of a gas turbine engine (as one, non-limiting, example) mayinclude an initial step 84 of flowing a first fluid through an innerflowpath 59″. A next step 86 includes exposing the second tube 58″ afirst thermal gradient. A next step 88 may include breaking the firstsacrificial support structure 86″ located between the tubes 56″, 58″ viathermal stress induced by the first thermal gradient. A next step 90 maybe simultaneous to step 84 and may include flowing a second fluidthrough the flowpath 74″. A next step 92 may include exposing the thirdtube 72″ to a thermal gradient. A next step 94 may include breaking asecond sacrificial support structure 68″ located between the second andthird tubes 58″, 72″.

Examples of additive manufacturing processes include, but are notlimited to, laser powder bed, electron beam melting, free formfabrication laser powder deposition and electron beam wire deposition,amongst others. Additive manufacturing systems include, for example,Additive Layer Manufacturing (ALM) devices, such as Direct Metal LaserSintering (DMLS), Selective Laser Melting (SLM), Laser Beam Melting(LBM) and Electron Beam Melting (EBM) that provide for the fabricationof complex metal, alloy, polymer, ceramic and composite structures bythe freeform construction of the workpiece, layer-by-layer. Theprinciple behind additive manufacturing processes may involve theselective melting of atomized precursor powder beds by a directed energysource, producing the lithographic build-up of the workpiece. Themelting of the powder occurs in a small localized region of the energybeam, producing small volumes of melting, called melt pools, followed byrapid solidification, allowing for very precise control of thesolidification process in the layer-by-layer fabrication of theworkpiece. These devices are directed by three-dimensional geometrysolid models developed in Computer Aided Design (CAD) software systems.

One example of an additive manufacturing system 100 capable ofmanufacturing the tube assembly 20 is schematically illustrated in FIG.6. The additive manufacturing system 100 has a build table 102 forsupporting the assembly 20 and generally holding a powder bed 104, aparticle spreader, wiper or sprayer 106 for spreading, spraying orotherwise placing the powder bed 104 over the manufacture portion of theassembly 20 and build table 102, an energy gun 108 for selectivelymelting regions of a layer of the powder bed, a powder supply hopper 110for supplying powder to the spreader 106, and a powder surplus hopper112. The additive manufacturing system 100 may be constructed to buildthe assembly 20, or any portions thereof, in a layer-by-layer fashion.The powder bed 104 is composed of the same material composition as theassembly being additively manufactured.

A controller 114 of the additive manufacturing system 100 may include acomputer 116 for entering data and that contains software forprogramming automated functions in accordance with inputted threedimensional computer aided design models of the assembly 20. The modelmay include a breakdown of the assembly 20 into a plurality of slices118 additively built atop one-another generally in a vertical orz-coordinate direction. Each solidified slice 118 corresponds to a layer120 of the powder bed 104 prior to solidification and each layer 120 isplaced on top of a build surface 122 of the previously solidified slice118. The controller 114 generally operates the entire system through aseries of electrical and/or digital signals 124 sent to the system 100components. For instance, the controller 114 may send a signal 124 to amechanical piston 126 of the supply hopper 110 to push a supply powder128 upward for receipt by the spreader 106. The spreader 106 may be awiper, roller or other device that pushes (see arrow 130) or otherwiseplaces the supply powder 128 over the build surface 122 of the assembly20 (or any portion thereof) by a pre-determined thickness that may beestablished through downward movement (see arrow 132) of the build table102 controlled by the controller 114. Any excess powder 128 may bepushed into the surplus hopper 112 by the spreader 106.

Once a substantially level powder layer 120 is established over thebuild surface 122, the controller 114 may send a signal 124 to theenergy gun 108 that energizes a laser or electron beam device 134 andcontrols a directional mechanism 136 of the gun 108. The directionalmechanism 136 may include a focusing lens that focuses a beam (seearrows 138) emitted from device 134 which, in-turn, may be deflected byan electromagnetic scanner or rotating mirror of the mechanism 136 sothat the energy beam 138 selectively and controllably impinges uponselected regions of the top layer 120 of the powder bed 104. The beam138 moves along the layer 120 melting region-by-regions of the layer 120at a controlled rate and power, melting each region into pools that thenform with, or sinter to, the adjacent build surface 122, solidify, andultimately form the next top slice 118. The process then repeats itselfwhere another powder layer 120 is spread over the last solidified slice118 and the energy gun 108 melts at least a portion of that layer alongwith a meltback region (i.e. sintering) of the previously solidifiedslice 118 to form a uniform and homogeneous assembly 20, or portionthereof.

It is understood that relative positional terms such as “forward,”“aft,” “upper,” “lower,” “above,” “below,” and the like are withreference to the normal operational attitude and should not beconsidered otherwise limiting. It is also understood that like referencenumerals identify corresponding or similar elements throughout theseveral drawings. It should be understood that although a particularcomponent arrangement is disclosed in the illustrated embodiment, otherarrangements will also benefit. Although particular step sequences maybe shown, described, and claimed, it is understood that steps may beperformed in any order, separated or combined unless otherwise indicatedand will still benefit from the present disclosure.

The foregoing description is exemplary rather than defined by thelimitations described. Various non-limiting embodiments are disclosed;however, one of ordinary skill in the art would recognize that variousmodifications and variations in light of the above teachings will fallwithin the scope of the appended claims. It is therefore understood thatwithin the scope of the appended claims, the disclosure may be practicedother than as specifically described. For this reason, the appendedclaims should be studied to determine true scope and content.

What is claimed is:
 1. A tube assembly comprising: a first tube defininga first flowpath along a centerline; a second tube generally spacedradially outward from the first tube, and wherein a first void islocated between and defined by the first and second tubes; and a firstsacrificial support structure located in the first void and extendingbetween the first and second tubes that is constructed and arranged tobreak when under normal operating stress thereby limiting thermalconduction between the first and second tubes.
 2. The tube assembly setforth in claim 1, wherein the first tube, the second tube and thesupport structure are additive manufactured as one unitary piece.
 3. Thetube assembly set forth in claim 1, wherein the first void is aninsulating void.
 4. The tube assembly set forth in claim 3 furthercomprising: a third tube generally spaced radially outward from thesecond tube, and wherein a second flowpath is located between anddefined by the third and second tubes.
 5. The tube assembly set forth inclaim 4 further comprising: a fourth tube generally spaced radiallyoutward from the third tube, and wherein a second void is locatedbetween and defined by the fourth and third tubes.
 6. The tube assemblyset forth in claim 5 further comprising: a second sacrificial supportstructure located in the second void.
 7. The tube assembly set forth inclaim 6, wherein the second void is an insulating void.
 8. The tubeassembly set forth in claim 3, wherein the first void is under anegative atmospheric pressure.
 9. The tube assembly set forth in claim7, wherein the first and second cavities are under a negativeatmospheric pressure.
 10. The tube assembly set forth in claim 1,wherein the sacrificial support structure has a thickness within a rangeof 0.004 to 0.008 inches.
 11. The tube assembly set forth in claim 4,further comprising: a second sacrificial support structure located inthe second flowpath.
 12. The tube assembly set forth in claim 1, whereinthe tube assembly is at least part of a fuel nozzle for a gas turbineengine.
 13. A tube assembly comprising: a first tube defining a firstflowpath along a centerline; a second tube generally spaced radiallyoutward from the first tube by a radial distance, and wherein a firstvoid is located between and defined by the first and second tubes; and afirst support structure located in the first void, and extending betweenand engaged to the first and second tubes, wherein the first supportstructure has a length greater than the radial distance, and wherein thefirst support structure lies in a plane that is normal to thecenterline.
 14. The tube assembly set forth in claim 13, wherein thefirst support structure is one of a plurality of first supportstructures spaced circumferentially from one-another.
 15. The tubeassembly set forth in claim 13, further comprising: a third tubegenerally spaced radially outward from the second tube, and wherein asecond flowpath is located between and defined by the third and secondtubes; a fourth tube generally spaced radially outward from the thirdtube, and wherein a second void is located between and defined by thefourth and third tubes; and a second support structure located in thesecond void.
 16. The tube assembly set forth in claim 13, wherein thetube assembly is additive manufactured as one unitary piece.
 17. Thetube assembly set forth in claim 1, wherein the first sacrificialsupport structure is constructed and arranged to break via thermalstress induced by a thermal gradient imposed on the second tube.